The present invention relates generally to packet-based communication systems wherein data packets including address data and message or control data or both are propagated about a network in accordance with address data in the packets. The invention particularly relates to a network switch which includes a forwarding database and a multiplicity of ports of which one is connected to a router. The invention is mainly intended to facilitate the insertion into a network of a switch which can respond to protocol addresses and be able to relieve the router of traffic which requires simple routing decisions.
As is well known, a data packet is typically formed in a relatively high level of a communications protocol and before it is transmitted from an originating device it has attached to it a header which includes address data. The address data normally includes a protocol or network address, defining a xe2x80x98subnetxe2x80x99 on which the destination station is located and usually also an identification of that destination station. The network layer or protocol address is employed by a xe2x80x98routerxe2x80x99, which term is intended to include devices which have a routing facility, to direct the packet to the appropriate subnet.
The address data within a packet needs to include at some stage a media access control address, otherwise known as a xe2x80x98layer 2xe2x80x99 or xe2x80x98data linkxe2x80x99 address. The media access control (MAC) address is employed by switches and other devices to determine, when forwarding a packet, the specific device to which the packet should be sent.
When a packet does not contain a media access control address, as when for example a first end station initially attempts to communicate with a destination end station, it is necessary to perform an address resolution protocol, wherein a packet is broadcast indiscriminately. An end station receiving, an address resolution packet (ARP packet) containing its protocol address can reply with its media access control address. That enables a switch to establish in a forwarding database an entry which relates the particular protocol address with a media access control address and typically a port number of a port to which packets addressed to that destination end station will be sent.
Some operations in response to address data in packets are comparatively simple and speedy. For example, xe2x80x98bridgingxe2x80x99 is the conventional term employed for responding to the MAC address and directing a packet to the device identified in that address. However, other forms of response, particularly xe2x80x98routingxe2x80x99 are more complex and require greater time. In particular, it is necessary to employ a router to perform such tasks as determining a best route for a packet to take, the prevention of indefinite looping of a packet, and a variety of other functions well known to those skilled in the art. Commonly, the performance of those ancillary functions is very much slower than the performance of a bridging function.
However, it is known to provide a switch which can operate both in xe2x80x98layer 2xe2x80x99 and xe2x80x98layer 3xe2x80x99, and which more particularly can in a default mode perform layer 2 look-ups but which can act also as a router, performing layer 3 look-ups. One example of such a switch, which operates with a single look-up table for both bridging and routing decisions, is disclosed in published GB patent application No. 2337674. Another example is a 12 port 100/1000 Mb/s Ethernet switch type 3C17700 made by 3Com Corporation. Such switches require configuration of their routing tables in order to operate in xe2x80x98layer 3xe2x80x99 but differ from fully functional routers in that the routing can be performed at high speed, e.g. xe2x80x98wire speedxe2x80x99. It is presumed in the following that the xe2x80x98network switchxe2x80x99 employed is a switch of this character, being capable of xe2x80x98layer 2xe2x80x99 switching and, with appropriate configuration, xe2x80x98layer 3xe2x80x99 switching. Such a switch does not perform all the functions which a software controlled router can normally perform. If such a network switch is employed for example in a local area network and a router is also provided to route packets between that local area network and (for example) a wide-area network it would be beneficial to offload from the router the basic routing function (layer 3 switching) within the local area network and in particular, irrespective of what kind of router is employed, to employ the layer 3 switching capability of the switch to route local traffic between subnets.
As will be apparent to those skilled in the art and as more particularly discussed hereinafter, a switch which is capable of xe2x80x98layer 3xe2x80x99 routine will include in its forwarding database entries which relate a protocol (IP) address, a MAC (media access control) address identifying the next hop of a package intended for the protocol address and an identification (such as a number or port mask) of the port to which a packet routed according to that entry must be sent by the switch. The action of establishing in a database an entry of that character (which may include an identification of a virtual local area network) is usually termed xe2x80x98learningxe2x80x99 the protocol address for a particular port.
It is known in itself to control a switch, for example remotely by way of a xe2x80x98managementxe2x80x99 port, so that protocol (layer 3) addresses cannot be xe2x80x98learnedxe2x80x99 for a particular port.
The main object of the invention is to facilitate the offloading of routing decisions from a router to an associated switch which is capable of switching decisions on both media access control address and protocol addresses. The main feature of the invention is to provide the connection between the router and the switch only by way of a port in respect of which the switch can learn media access control addresses but is unable (for example by being specifically disabled) to learn protocol (IP) addresses.
Further advantages and features of the invention will become apparent from the following detailed description with reference to the accompanying drawings.